Objectives: Beta-amyloid (Aβ) peptides are most toxic to cells in oligomeric form. It is
commonly accepted that oligomers can form ion channels in cell membranes and allow calcium and
other ions to enter cells. The activation of other mechanisms, such as apoptosis or lipid peroxidation,
aggravates the toxicity, but it itself can result from the same initial point, that is, ion disturbance due to
an increased permeability of membranes. However, experimental studies of membrane channels
created by Aβ are surprisingly limited.
Methods: Here, we report a novel flow cytometry technique which can be used to detect increased
permeability of membranes to calcium induced by the exposure to amyloid peptides. Calcium entry
into the liposome is monitored using calcium-sensitive fluorescent probe. Undamaged lipid membranes
are not permeable to calcium. Liposomes that are prepared in a calcium-free medium become able to
accumulate calcium in a calcium-containing medium only after the formation of channels.
Results: Using this technique, we demonstrated that the addition of short amyloid fragment Ab25-35,
which is known for its extreme toxicity on cultured neurons, readily increased membrane permeability
to calcium. However, neither similarly sized peptide Ab22-35 nor full-length peptide Aβ1-42 were producing
channels. The formation of channels was observed in the membranes made of phosphatidylserine,
a negatively charged lipid, but not in membranes made of the neutral phosphatidylcholine.
Discussion: We have analyzed several issues which could be critical for understanding the pathogenesis
of Alzheimer’s disease, specifically 1) the need for a negatively charged membrane to produce the
ion channel; 2) the potential role of the aggregated form in cellular toxicity of Aβ peptides; 3) channelforming
ability of multiple degradation products of amyloid; 4) non-specificity of ion channels formed
by amyloid peptides. Potential targets of channel-forming oligomers appear to be intracellular and are
organelles well-known for dysfunction in Alzheimer’s disease (mitochondria and lysosomes). In fact,
lysosomes can also be the producers of degraded amyloid. Provided speculations support the hypothesis
that neuronal toxicity can be caused by the degradation products of beta-amyloid.